BACKGROUND: Fat mass (FM) and fat-free mass (FFM) in early life are associated with later obesity and cardiometabolic disease. OBJECTIVES: This study aimed to assess the associations of FM and FFM at birth and conditional FM and FFM accretion from 0 to 5 y with kidney outcomes at the 10-y follow-up. METHODS: The Ethiopian Infant Anthropometry and Body Composition birth cohort included term infants born in Jimma town, with a birth weight ≥1500 g, and having no congenital malformations. Air-displacement plethysmography was used to measure body composition. Serum cystatin C was determined and kidney dimensions were assessed by ultrasound when children were aged ∼10 y. Conditional growth modeling was used to compute FM and FFM accretion between different time points over 0-5 y. Multiple linear regression analysis was used to examine associations of birth FM and FFM and conditional FM and FFM accretion in selected age periods with serum cystatin C and total kidney volume at the 10-y follow-up. RESULTS: A total of 350 children were followed up at a mean age of 9.8 (±1.0) y. A 1 standard deviation (SD) higher conditional FFM accretion from 3 to 6 mo was associated with 7.6% [95% confidence interval (CI): 1.9%, 13.0%) lower serum cystatin C but higher conditional FFM accretion 48-60 mo was associated with 5.3% (95% CI: 1.9%, 9.0%) higher serum cystatin C. A 1 SD higher conditional FM accretion in the periods 6-48 mo and 48-60 mo was associated with β = 7.7 (95% CI: 4.8, 10.7) and β = 6.4 (95% CI: 1.6, 11.1) cm3 greater kidney volume, respectively. A 1 SD higher birth FFM and FFM accretion in the periods 3-6 mo, 6-48 mo, and 48-60 mo was associated with β = 4.7 (95% CI: 2.1, 7.2), 14.1 (95% CI: 6.3, 22.0), 4.2 (95% CI: 0.9, 7.4), and 7.1 (95% CI: 2.5, 11.7) cm3 greater kidney volume, respectively. CONCLUSIONS: A higher conditional FFM gain in age from 3 to 6 mo results in better kidney function at the 10-y follow-up, whereas a higher conditional FFM gain in age from 4 to 5 y results in a lower kidney function. Kidney volume at the 10-y follow-up is associated with higher birth FFM and higher conditional FM or FFM growth in most growth periods.